The present invention relates generally to voltage converters that output multiple voltages, and more particularly to protecting a linear post-regulator used in such a converter against an overcurrent condition.
As herein relevant, a voltage or power converter is a circuit or system that receives an input voltage (Vin) that is AC in an AC:DC voltage converter, or DC in a DC:DC voltage converter, and generates at least two voltages that are provided as rectified DC outputs (Vo, VoAUX). Typically the Vo voltage is sampled and fed back to the voltage feedback node (VFB) of a pulse width modulator (PWM) whose output can control magnitude of Vo. However the VoAUX voltage is neither sampled or used to control the PWM, nor otherwise voltage regulated. Instead, a linear regulator is used between the relevant converter output and the VoAUX node as a post-regulator.
Such converters may be implemented in a variety of topologies. By way of example, FIG. 1A depicts a prior art isolation-providing DC:DC voltage converter 10. Converter 10 provides isolation in that the input ground is separate from the output ground. However for use with the present invention, it is not critical whether a converter does or does not provide isolation.
In the exemplary topology of FIG. 1A, converter 10 receives a source of input potential Vin on the system input side 20, and converts Vin to a Vo potential and a VoAUX potential on the system output side 30. Loads will be coupled between the Vo node and output-side ground, and between the VoAUX node and output-side ground. Other systems 10 could of course generate more than two output voltages, and if system 10 were an AC:DC converter, then Vin could represent a raw input AC voltage that has been rectified to yield Vin. As noted, while FIG. 1A shows an isolating converter having separate input-side ground and output-side ground, converter 10 is merely exemplary, and could in fact be non-isolating, with a common ground for the system input-side and output-side.
In the exemplary topology of FIG. 1A, transformer T1 provide isolation between the input and output sides of system 10, as does isolator unit 11. Transformer T1 typically comprises at least one primary winding W1 and at least one secondary winding W2, shown here as being tapped, from which raw output voltages Vo and VinAUX will be provided. The input side of converter 10 includes a switch Q1 coupleable in series between one end primary transformer winding W1 and input-side ground (or other input-side reference potential). If additional primary side windings are present, each such winding could also have a switch, and be similarly coupleable. However, it is not required that converter 10 provide isolation, in which case I1 could be omitted, and input side ground and output side ground would be a common ground.
Referring to FIG. 1A, in a fashion well known to those skilled in the relevant art, switch Q1 opens and closes upon receipt of drive signal from a drive circuit 40. In turn, circuit 40 outputs the drive signal in response to input signals from a pulse width modulator (PWM) 50 that operates preferably in response to a feedback sample (kxc2x7Vo) taken from output voltage Vo, e.g., via resistor string R1 and R2. Voltage Vo is output from a rectifier circuit, here shown as a simple diode-capacitor, D1 and C1. Commonly the kxc2x7Vo sample is coupled to a voltage feedback node (VFB) on PWM 50. A source of Vbias (not shown) is coupled to provide operating potential for PWM 50.
In operation, the kxc2x7Vo sample at the VFB node is compared within PWM 50 to a stable reference voltage (not shown). PWM 50 then generates an appropriate correction signal based upon the voltage difference between kxc2x7Vo and the reference potential. The correction signal is suitably coupled, e.g., via an isolator I1 if required, to driver 40 to command switch Q1 in a corrective fashion. For example, if PWM 50 determines that Vo is too low, the correction signal from the PWM can cause switch Q1 to turn-on with increased pulse width, to increase duty cycle and thus magnitude of Vo. Or, if the PWM determines that Vo is too high, the PWM will cause drive circuit 40 to turn-on Q1 with decreased pulse width, to decrease duty cycle and thus magnitude of Vo.
When switch Q1 turns-on, Vin is impressed across input winding W1, and essentially Vin is sampled or chopped. The resultant chopped signal is inductively coupled via transformer T1 to the secondary transformer winding W2. On the output side of system 10, diode D2 and capacitor C2 filter the chopped AC to yield raw potential VinAUX, which is coupled as input to a post-linear regulator circuit 60 to yield VoAUX. Internal to regulator 60 is a feedback loop 70 that is used to limit the maximum current available from the VoAUX node.
System 10 in FIG. 1A is typical of many prior art converters in that the Vo voltage can be well regulated by feedback to the PWM, but there is no real regulation of the potential VoAUX, only a limit as to maximum current at the VoAUX node. For example, the PWM may control magnitude of Vo to within about xc2x12%, whereas VinAUX may vary xc2x15% to xc2x110% or so, as the magnitude of Vin andlor loads on either output node vary. Generally there is but one PWM in a converter system, and the VoAUX node simply is not voltage regulated using PWM feedback. A post-linear regulator can regulate VoAUX to within about xc2x12%. But protecting post-regulator 60 against thermal overload can be a challenge, especially if regulator 60 is implemented with discrete components, rather than as a single IC. For example, if the load resistance LOADAUX becomes too low, or even a short circuit, regulator 60 must stand-off a voltage differential of (VinAUXxe2x88x920) and a maximum value of load current IAUX. The pass device must dissipate the power equal to the product of the stand-off voltage and maximum current, and can readily be damaged. Some prior art topologies include current foldback to reduce magnitude of output current under short-circuit load conditions, but such topologies still do not use input-to-output feedback to voltage regulate the VoAUX node potential.
FIG. 1B depicts an exemplary linear post-regulator 60, a circuit that will be coupled in series between VinAUX and VoAUX to limit the maximum permissible load current (IAux) available to LOADAUX. Regulator 60 includes a pass element, here a bipolar transistor Qpass used as an emitter follower, coupled in series with a current sensor 70, through which current IAUX passes.
Regulator 60 further includes a first amplifier 80 that compares a sensor 70 measure of IAUX with a reference voltage 90 representing a maximum threshold current. Regulator 60 also includes a second amplifier 100 that compares a measure of VoAUX potential to a reference potential 110. A feedback loop 120 is provided such that the magnitude of the input or control signal to pass element Qpass is a function of the magnitude of sensed current IAUX. In the example shown in FIG. 1B, Q1 is a bipolar transistor whose input signal is the base-emitter drive voltage established by amplifier 100. If sensor 70 determines that IAUX is exceeding a threshold set by reference 90, the effect of the feedback in the regulator is to decrease the Qpass base-emitter voltage, thus decreasing collector and emitter current, or IAUX. Diode Dr in feedback loop 120 protects amplifiers 80 and 100 from damage from each other""s output signals.
An exemplary current sensor 70 is shown in FIG. 1C. Sensor 70 can include a small impedance sense resistor Rs across which IAUX creates a voltage drop proportional to Rsxc2x7IAUX. This potential is sensed with a differential amplifier Ae whose output is coupled to amplifier 100. If IAUX increases sufficiently, the output signal from the error amplifier Ae will exceed the threshold level set by reference 90, whereupon Qpass will receive less base-emitter drive. It is understood that FIG. 1C is exemplary and current flow IAUX can be sensed using other circuits and other techniques.
As noted above, post-linear regulator 60 can indeed limit the magnitude of the output current IAUX. But under worst case conditions, if LOADAUX resistance is very small or even zero1 Qpass will be required to safely dissipate power equal to VinAUXxc2x7IAux. By way of example, if VinAUX is 70 V and VoAUX is nominally 60 V with IAUX≈1 A, under normal conditions Qpass will dissipate 10 W, e.g., the product of 10 V and 1 A. But under short-circuit output load conditions, Qpass will be required to dissipate approximately 70 W, e.g., (70 V-0V)xc2x71 A. If post-regulator 60 is fabricated on a single integrated circuit (IC) die, reasonably adequate thermal protection can often be provided. But in many converter systems, the post-regulator will be implemented with discrete components and it can be difficult to adequately protect the pass element against thermal stress due to a short circuit or very low LOADAUX resistance.
Thus, for use with a converter that outputs at least two voltages, one of which is voltage-regulated with a PWM and feedback, and one of which simply has a linear post-regulator to limit current, there is a need for a topology that provides some measure of feedback voltage regulation for the current limited node. Further, there is a need to protect even a discretely implemented post-linear regulator with thermal protection, preferably such that the regulator pass element dissipates minimum rather than maximum power under short circuit load conditions to that node.
The present invention provides such topology.
As noted, prior art voltage converters that output Vo and VoAUX provide output-to-input PWM system feedback to regulator voltage at the Vo output node, but use only a post-regulator with local feedback to limit current delivered from the VoAUX node, and do not control VoAUX with output-to-input feedback. By contrast, the present invention regulates the Vo output node with similar output-to-input PWM system feedback, but also regulates potential at the VoAUX node using feedback from a linear post-regulator to a node of the same PWM. The linear post-regulator, which is in series between the converter VinAUX node and the VoAUX node, limits output current from the VoAUX node and is protected by topology according to the present invention against excessive dissipation under short-circuit load condition. When excessive current is drawn from the VoAUX node, feedback from the post regulator to the PWM causes a reduction in output voltage at the Vo node and at the VinAUX node. However as the VoAUX node current reduces to an acceptable level, feedback from the post-linear regulator to the PWM reduces, until the PWM is relatively unaffected by feedback from the regulator. Converter topology according to the present invention is such that when current drawn from the VoAUX node exceeds a maximum current level, dissipation across the linear post-regulator is actually at a minimum. Thus, thermal protection is provided even if the linear post-regulator is implemented with discrete components.
When the current drawn from the VoAUX node is excessive (e.g., exceeds a predetermined threshold) and requires limiting, the linear post-regulator provides feedback to at least one input node of the same PWM that provides output-to-input feedback control over the Vo node voltage. Feedback from the post-linear regulator can go to the PWM COMP node, the SoftStart node, the VFB node, among other PWM nodes. This PWM feedback will reduce the VinAUX node potential, and also VoAUX and Vo potential.
In the present invention, when maximum current limiting occurs in the linear post-regulator, the regulator pass device is operated at or close to saturation. As a result, although the pass device now conducts the maximum limit current, the voltage across the input-output terminals of the device (e.g., collector-to-emitter for a bipolar device) is at a minimum, e.g., Vce sat. The product of the maximum current and the low saturation voltage represents a low level of dissipation that the pass device can safely handle. Indeed, thermal dissipation in the regulator pass device is actually least when current limiting is invoked, due to saturation mode operation of the pass device.
Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings.